The trend in automotive electrical systems has always been towards more power and higher voltages. At this time, an element of the trend involves the combination of the alternator and starter into a single IC engine driven unit. This combined starter/alternator can be driven either directly on the crankshaft of the IC engine as a part of the flywheel on one end or the balancer on the other. Alternatively, the starter/alternator can be mounted for gear, belt, or chain drive from the crankshaft of other IC engine driven component (waterpump/A/C compressor/power steering pump, etc.)
The starter/alternator has become more powerful not only for increasing power (current and voltage) but also for more rapid and more frequent starting cycles of the IC engine as enhanced operating efficiencies are sought. In pursuit of these goals, the starter/alternator has become more sophisticated in its control systems and its responsiveness to system requirements for both starter functions and generating functions. In older fixed output generators (i.e., 1000 watts in a range of pre-selected engines speeds), the excess load would simply divert to the battery, or other electrical power storage device, similarly excess output would also divert. [Either of these eventualities led to either dimming lights and decreasing performance of electrical components, in the case of too small an alternator, or boiled away battery fluids in the case of too large a charging capacity!] However, in the event a starter/alternator is not controlled properly, the combined starter/alternator is generally sufficiently load responsive and powerful to either stall an IC engine, that is operating at low or idle speeds, or can also over speed itself in start up modes when the IC engine fails to attain self sustaining operation. And, even if neither of these circumstances occur, the starter/alternator responding to an increasing load can rapidly deliver too much current leading to overload and failure.